01 - Preheat your oven to 400°F.
02 - In a large skillet over medium heat, melt the butter. Add onions, carrots, celery, and potatoes (if using). Sauté for 5–7 minutes until the vegetables have softened.
03 - Stir in the minced garlic and cook for 1 minute until fragrant.
04 - Sprinkle the all-purpose flour over the sautéed vegetables and stir to coat thoroughly. Cook for 1–2 minutes, stirring continuously.
05 - Gradually whisk in the chicken broth and whole milk. Continue stirring until the mixture thickens to a creamy consistency, which should take about 3–5 minutes.
06 - Stir in the salt, black pepper, dried thyme, and dried parsley. Incorporate the cooked chicken and frozen peas into the sauce. Simmer for 2–3 minutes, then remove the skillet from the heat.
07 - Pour the prepared chicken and vegetable filling into a 9-inch pie dish or baking dish.
08 - Carefully roll out the ready-made pie crust to fit over the dish. Place the pastry over the filling, pressing the edges firmly to seal. Trim any excess pastry.
09 - Cut a few small slits into the top of the pastry to allow steam to escape during baking. Brush the entire surface of the crust with the beaten egg for a golden finish.
10 - Bake for 30–35 minutes, or until the pastry crust is golden brown and the filling is bubbling vigorously.
11 - Allow the chicken pot pie to rest for 10 minutes before serving to ensure the filling sets properly.